The invention relates to a steering control system for a tracked vehicle.
Tracked vehicles are steered by driving one track faster or slower than the other. Most tracked vehicles with differential steering have a steering pump which is driven by the engine rather than by the ground, so that the vehicle can turned without forward or reverse translational vehicle motion, and so that a more reasonably sized pump can be used. On currently available tracked vehicles, such as the 8000T tractor manufactured by John Deere and on tracked vehicles manufactured by Caterpillar, the steering wheel is spring centered so that there will be no vehicle turning unless the operator intentionally commands the vehicle to turn by rotating the steering wheel away from its spring-centered position. Such a system is described in U.S. Pat. No. 5,948,029, issued Sep. 7, 1999 to Straetker, and assigned to the assignee of this application.
A non-spring-centered steering input device is described in pending U.S. patent application Ser. No. 08/991,961, filed Dec. 17, 1997 (Atty. Docket No. 14524-US), and assigned to the assignee of this application. This non-centered steering input device makes possible a steering function which performs similar to the steering function of a wheeled vehicle. A primary characteristic of a wheeled vehicle steering function is that rotation of the steering wheel commands a particular curvature of the vehicle's path, and the amount of curvature is largely independent of the engine speed, wheel speed, vehicle weight, or ground conditions. This characteristic is achieved for a tracked vehicle by sensing wheel speed and steering motor speed and controlling the relative speed of the tracks in proportion to the wheel speed. If the vehicle is moving quickly, the tracks have a large relative motion, and this relative motion is reduced as the vehicle slows down. When the vehicle stops forward or reverse longitudinal motion, track relative motion also stops. However, with such a steering input device, when the vehicle is performing a sharp turn and the transmission is disengaged, either by depressing the clutch or shifting to neutral, it is possible for tractor momentum to continue vehicle motion, or the steering motor may actually propel the vehicle through this sharp turn. The control system will sense the longitudinal motion of the vehicle and command the proper track relative motion to maintain the turn radius. However, when the track relative motion is the actual cause of the longitudinal motion, then it is possible for the vehicle to inadvertently continue turning until the operator intervenes--a self-perpetuating turn. It would be desirable to provide a steering control system which prevents or limits self-perpetuating turns.